The difficulty of fabricating efficient linear optical couplers on an optical fiber bus with highly controlled coupler ratios involves the complete removal of the cladding that surrounds the core of the optical fiber bus at the coupler site as well as the precise placement of components at the coupler site. The size, shape, position, and cleanliness of the cladding removal is critical in making a good linear optical coupler. Similar problems exist for the fabrication of optical mode scramblers. Further, it is important that the optical fiber core not be damaged during the cladding removal, and that the fabrication method have a low cost per coupler or mode scrambler.
Mechanical stripping methods are known for fabricating coupler openings or mode scrambler patterns in the cladding. These methods physically scrape the cladding from the core; however, such stripping often does not achieve the correct opening size and shape, and frequently damages the core at the coupler or mode scrambler site, thus reducing the efficiency and strength of the optical fiber bus. Also, mechanical stripping techniques often leave cladding particles at the coupler site, and further, do not allow for precise placement of components at the coupler site.
In addition to mechanical stripping, chemical etching has been utilized to remove the cladding from the optical core. Chemical etching techniques are limited because of difficulties in achieving the correct opening size and shape, in accurately positioning the site defining mask, and in controlling the etching.
U.S. Pat. No. 4,626,652 discloses a method for fabricating optical couplers that uses a laser to remove all but a thin layer of the cladding from two fibers and then uses heat to fuse the two fibers together. If the cladding normally propagates the energy from the laser, the method requires the cladding to be treated or coated with a substance, such as a dye, to prevent the laser beam from penetrating the core of the fiber. Further, the laser beam disadvantageously scans back and forth across the region to prevent total removal of the cladding.
Mode scramblers have been fabricated in various ways. In U.S. Pat. No. 4,676,594, a mode scrambler is achieved by making a deformation, such as a groove or a notch, on one side of the optical fiber bus orthogonal to the longitudinal axis. The patent discloses etching or mechanical machining that produces the groove or notch in the optical fiber cladding.
Another optical fiber mode scrambler is disclosed in, for example, the articles "Measurement of Baseband Frequency Response of Multimode Fiber By Using A New Type of Mode Scrambler" by M. Tokuda et al. in "Electronic Letters," Mar. 3 1977, Vol. 13, No. 5 at pages 146-147; and "Transmission Characteristics Of Multimode W-type Optical Fiber: Experimental Study Of The Effect Of The intermediate Layer" by K. Mikoshiba et al. in "Applied Optics," Vol. 17, No. 17, Sept. 17, 1978, at pages 2836-2841. In these articles the disclosed mode scrambler includes an optical fiber comprising specific sinusoidal serpentine bends.
Another form of an optical fiber mode scrambler is disclosed in, for example, the articles "Mode Scrambler For Optical Fibers" by M. Okeda et al. in "Applied Optics," Apr. 1977, Vol. 16, No. 4, at pages 1045-1049; and "Mode Scrambling Can Enhance Fiber Optic System Performance" by S. L. Storozum in "Electronics," Feb. 24, 1981, Vol. 4, No. 54, at pages 163-166. The mode scrambler in these articles comprises a few fibers in a bundle surrounded by a heat shrinkable tube which when shrunk causes fiber microbending.
Star couplers for providing mode scrambling are also well known and comprise a first plurality of fibers interconnected to a second plurality of fibers by means of a mixing element either comprising a planar waveguide section or formed by twisting and fusing the twisted waveguides together. In this regard see, for example, U.S. Pat. Nos. 4,362,357 and 4,449,783 or Fundamentals Of Optical Fiber Communications by M. K. Barnoski, 2nd Edition, Academic Press, 1981, at pages 337-339.
U.S. Pat. No. 4,749,248 discloses devices for coupling optical power from and/or into optical fiber. This patent discloses the use of a photorefractive effect to change the refraction index of the core of a single mode fiber in order to transfer energy from bound modes to so-called tunneling leakly nmodes (TL). The TL modes can then be removed from the cladding of the fiber guide with the aid of a dielectric body in contact with the fiber at a downstream point that has a refractive index approximately equal to or greater than index of refraction of the cladding. The photorefractive effect requires that the core be doped with an appropriate dopant (e.g., Fe or Bi) and constantly exposed to light of the appropriate wavelength. If the light is removed, the index of refraction returns to its original state. In addition, this patent proposes the use of a photoelastic effect to vary the refractive index of the core using a light that induces stresses in the core.
The problem that exists in the art is to provide a technique for making effective couplers and mode scramblers on an optical fiber bus by removing the cladding from the optical core without damaging the core and also maintaining the proper size, shape, and accuracy of position.